Electron tube apparatus



Jan. M, 1936. R. P. BALLOU ZMEKW ELECTRON TUBE APPARATUS Filed Feb. 26,1.932 3 Sheets-Sheet l v 1 L I 7 ll (20 INVENTOR I Richard P; Ballou BYQKW HIS ATTORNEY Jam 1936- R. P. BALLOU 2,027,50

ELECTRON TUBE APPARATUS Filed Feb. 26, 1932 3 Sheets-Sheet 2 INVENTOR 27 lZiehanoZ RBaZZou.

HIS ATTORNEY Jan. M, 1936.. R, P. BALLOU ELECTRON TUBE APPARATUS mzmwFiled Feb. 26,

1932 5 Sheets-Sheet 5 INVENTOR HzdzaPd 1 Ballou BY Z 5- 55 412W H I5ATTORNEY Patented Jan. 14, 1936 UNITED STATES PATENT OFFICE ELECTRONTUBE APPARATUS Application February 26, 1932, Serial No. 595,319

12 Claims.

My present invention relates to a method oi? controlling the mutualcharacteristic of an electron tube circuit, and to means for carryingout the new method. More particularly, the invention comprises a methodfor straightening out the curved mutual characteristic of a vacuum tube,thus gaining an increase in the over-all efficiency and output of avacuum tube amplifier by permitting operation without distortion over awider range of the tubes mutual characteristic than hitherto foundpossible with a particular type of circuit.

The mutual characteristic of a vacuum tube having three or more elementsis a curve representing the relation between applied grid voltage andoutput plate current. It is said to be static if the plate voltage iskept constant, or dynamic if, as in usual operating conditions, thebattery voltage remains constant and the plate voltage decreases withincreasing positive grid excitation. Such curves are ordinarilysubstantially linear over a relatively short portion of their length,corresponding to a relatively small range of applied grid voltage. Thedynamic curves are generally linear over a relatively greater range thanthe static curves. While various operating conditions may imply thedynamic condition the method of compensation hereinafter disclosed isequally applicable to either condition. When used as an audio amplifier,the operation of the tube has had to be limited to this relatively smallrange of applied grid voltage to prevent distortion of the applied gridvoltage, introduced into the output by the curvature of the mutualcharacteristic. One object of the present invention is thus to provide amethod of, and means for, increasing the available range of operation ofan audio amplifier in a class A circuit. Another object is to improvethe operation of a push-pull class B amplifier by providing a moredefinite cut-off point on the lower end of the mutual characteristic.

Broadly the method comprises the compensation of one or both ends of themutual characteristic of a tube by the addition of a voltage in the gridcircuit which voltage is a continuous function of the excitation voltageand which is so superimposed thereupon as to leave the midpoint of thecharacteristic unchanged while applyin gradually increasing compensationtoward one or both of the extremities thereof; the superimposed negativevoltage acting to decrease the positive potential and to increase thenegative potential applied to the grid by the excitation voltage in sucha manner as to increase the range of the linear portion of the mutualcharacteristic. Preferably the compensating negative voltage for thegrid of the tube whose mutual characteristic is to be compensated varieswith the plate current of a second tube to the grid of which theexcitation voltage is alsoapplied, for, in this manner, the distortionof the applied grid voltage may be made just sufiicient to counteractthe distortion introduced by the tube, with the result that the outputplate current is substantially proportional to the excitation voltageover a wide range. The method may be applied to either or both ends ofthe mutual characteristic as desired and, should such a result bedesired, could be so applied as to increase, rather than eliminate, thecurvature of the mutual characteristic.

For a better understanding of the invention reference may be had to theaccompanying drawings of which: Fig. 1 is a simple circuit illustratingthe preferred method of correcting the curvature of the upper end onlyof the mutual characteristic. Fig. 2 is a diagram of various voltage andcurrent wave forms explanatory oi the operation of Fig. 1. Fig. 3 is acircuit of the type of Fig. 1 but arranged for correcting the lower endof the mutual characteristic. Fig. 4 is a circuit illustrating analternative method of correcting the curvature of the upper end of themutual characteristic, basically diiferent. Fig. 5 is a circuitcombining features of the circuits of Figs. 1 and 3 to straighten outboth ends of the mutual characteristic. Fig. 6 is a circuit arranged tocompensate both ends of the characteristics of two tubes of a push-pullstage of audio amplification. Fig. '7 is a circuit combining features ofFigs. 3 and 4 for compensating both ends of the characteristic of asingle audioamplifier. Fig. 8 is a circuit illustrating the use of afull wave rectifier and a single compensating tube to improve theoperation of a push-pull amplifier. This circuit is equivalent to thatof Fig. 6 in performance. Fig. 8a is a fragmentary view of Fig. 8showing the connections of the rectifier for a class A amplifier. Fig.8b is a fragmentary View of Fig. 8 showing the connections of therectifier for a class B amplifier. Fig. 9 is a circuit similar to thatof Fig. 8 but including means for applying greater compensation to oneend of the mutual characteristic than to the other end. As shown, thiscircuit is equivalent to that of Fig. 5 in performance.

In Fig. 1, the amplifier I whose characteristic is to be compensated isshown with its plate connected through a suitable source of potential 2with a load device 3. Device 3 may be coupling to a succeeding amplifieror may represent a loud speaker or some other load. An input transformer4 having a secondary 4a for supplying the excitation voltage to the gridof tube I and a secondary 4b for supplying excitation voltage to thegrid of a compensating tube 5 has its primary connected across the inputterminals 6 of the circuit. A resistor I in the plate circuit of tube 5is so connected in the grid circuit of tube I as to cause the potentialdifference thereacross due to current flowing in the plate circuit oftube 5 to increase the negative bias of the grid of tube I; fixednegative bias for tube I being provided by a battery 8 and for tube 5 bya battery 9.

With the above described circuit if a voltage wave of the formillustrated at I0 (Fig. 2) is impressed upon the transformer 4, themutual characteristic of tube 5 will distort this wave into the formillustrated at II which results in a voltage drop across resistor Iillustrated at I2. The potential difference across resistor I beingopposite to that across secondary 4a, the wave impressed upon the gridof tube I will take the form illustrated at I3; being the differencebetween the wave forms I0 and I2. Tube I will distort this impressedvoltage in the same way that tube 5 distorted the original voltage wave,with the result that the output current wave will have the formillustrated at I4, which is similar to the impressed wave I0. In Fig. 2no amplification is illustrated for simplicity and the transformer isassumed to be distortionless. Only the positive half cycle of thevarious waves has been illustrated in Fig. 2 as it is only during thisportion of the cycle that the characteristic of tube I will becompensated. Thus the circuit of Fig. 1 increases the range of operationof the audio amplifier I by superimposing upon the excitation voltagethe negative voltage due to the potential difference across resistor Iwhich is a continuous function of the excitation voltage and thus ineffect, extending the upper extremity of the linear portion of themutual characteristic. The circuit thus makes possible the operation ofthe tube with grid voltages of greater amplitude with correspondingincrease in efliciency and power output. 1

If, for any reason, it is desired to increase the curvature of the upperend of the mutual characteristics only a slight change in the circuit ofFig. 1 would be required for this purpose. This change involves only thereversal of the connection to resistor I; that is, the battery 8 wouldbe connected to the end of resistor I connected to the cathode of tube 5and the upper end of secondary 4a would be connected to the end ofresistor I connected through the plate battery I5 with the plate of tube5. This might be desirable in case the mutual characteristic of thecircuit were linear-usually a sign of approaching saturation.

If the lower end rather than the upper end of the characteristic of tubeI is to be compensated the circuit of Fig. 3 may be employed. Fig. 3differs from Fig. 1 only in the substitution for the transformer 4 ofresistance coupling to the input terminal 6 by a resistor I6, and in theconnections of the compensating tube 5 which, in Fig. 3, is arranged forthe application of positive potential to the biasing battery 9 of tube 5during the half cycle when negative potential is applied to the biasingbattery 8 of tube I; for this purpose the grid of tube 5 is connectedthrough battery 9 with a tap along resistor I6 and the cathode of tube 5is connected to the end of resistor IE to which the grid of tube Iconnected through battery 8 and resistor I. With this arrangement whenthe end of resistor I5 connected to the cathode of tube I is positivewith respect to the other end thereof, current will flow through theplate circuit of tube 5 and cause the potential difference acrossresistor I to increase the negative potential applied to the grid oftube I, thus compensating the lower end of the characteristic by theaddition of a variable negative voltage to the excitation voltage duringthe negative half cycle thereof. In audio amplification this circuit ofFig. 3 is of particular value.

Obviously either transformer or resistance coupling to the inputterminal 6 could be employed with the circuits of Figs. 1 and 3 asdesired.

In Fig. 4 is illustrated an arrangement for compensating the upper endof the mutual characteristic of tube I which does not require theprovision of the compensating tube 5. In Fig. 4 a compensating resistor20 is inserted in the plate circuit of tube I and the potentialdifference across this resistor due to current fiowing therethrough issuperimposed upon the excitation voltage of the tube to add a negativevoltage thereto during the positive half cycle of the excitationvoltage, which negative voltage continuously increases as the excitationvoltage approaches its peak. For this purpose one end of the secondaryof the input transformer ZI is connected to the cathode of tube Ithrough resistor 20; the other end thereof being connected to thecontrol grid of the tube. The grid biasing battery 9 may be included inthe grid connection as shown, or, if desired, grid bias may be providedby shunting a portion of resistor 20 with a large capacity condenser.With the circuit above described compensation is effected not only inthe input circuit but, to a certain extent, in the plate circuit asWell. The distortion of the excitation voltage by the potentialdifference across resistor 20 will not be such as to give a completelinear relationship between the excitation voltage and the plate currentbut will substantially increase the eificient operating range of thetube and decrease the distortion introduced thereby. The circuit issimpler than that of either Fig. 1 or Fig. 3 in not requiring theprovision of a compensating tube but is not as satisfactory ifabsolutely constant plate voltage is required and, as mentionedheretofore,

cannot give as complete compensation as is given when a compensatingtube is employed. In this case the plate circuit must be resistive,rather than highly inductive, so that there is actually a change inplate current throughout each cyclic change in grid voltage.

In Fig. 5 a circuit of the type of Figs. 1 and 3 but arranged forcompensating both ends of the characteristic of tube I is indicated. Inthis circuit two compensating tubes 5 and 5a are used. The grid of tube5 is connected through the biasing battery 9 with one end of theresistor I5 connected across the input terminals 6. The grid of tube 5ais connected through the grid biasing battery 9a with a tap alongresistance IS. The cathodes of tubes 5 and 5a are connected to a secondtap on resistance I6 between the grid connections thereto and the gridof tube I is connected through grid biasing battery 8 and resistor Iwith the last mentioned tap on resistor I6. Resistor I is common to theplate circuits of both tube 5 and. tube 5a. With this arrangement duringthe half cycle, when the end of resistor I6 connected to the grid oftube 5 is positive with respect to the other end of resistor I6, thecurrent through the plate circuit of tube 5 causes a potential dropacross resistor i in a direction to decrease the positive grid potentialapplied between battery 8 and the cathode of tube I, which is connected,of course, to the other end of resistor I6. The upper end of thecharacteristic of tube I is thus compensated by tube 5. During the otherhalf cycle of the input wave, the potential applied to the grid of tube5a will cause a potential drop across resistor I in a direction toincrease the negative potential applied to the grid of tube I and thuscompensate the lower'end of the characteristic of this tube. Theresistor I6 could, of course, be replaced by an input transformer, ifdesired.

In Fig. 6 double end compensation of two tubes of a push-pull stage ofamplification is illustrated. In Fig. 6 two tubes I and la. whosecharacteristics are to be straightened are arranged with their controlgrids connected through the respective biasing batteries 8 and 8a toopposite ends of the secondary Ila of an input transformer IT to themid-point of which the cathodes of tubes I and la are connected throughresistor l. The compensating tubes 5 and 5a are connected across anadditional secondary winding iii of transformer II; their control gridsbeing connected through the biasing batteries 9 and 9a respectively withopposite ends of the winding IE; and their cathodes connected to themidpoint thereof. The plate battery I5 serves in common for tubes 5 and5a, its positive terminal being connected through resistor I with theplates of these tubes, and its negative terminal being connected withthe cathodes of the tubes. Similarly the plate battery 2' serves incommon for the plate circuits to tubes I and Ia. An output transformerfor the push-pull amplifying stage comprising the tubes I and la isindicated at I9.

With the arrangement of Fig. 6 during the half cycle when the upper endof the secondary of transformer I1 is positive with respect to the lowerend, tube 5 operates to decrease the positive potential applied to tubeI and toincrease negative potential applied to tube Ia. During the otherhalf cycle tube 5a operates to decrease the positive potential appliedto tube In. and to increase the negative potential applied to tube I.Thus the circuit operates to compensate both ends of the characteristicsof both tubes I and Fa.

Very satisfactory double end compensation can be obtained with thesimple circuit of Fig. '7 which combines features of the circuit ofFigs. 3 and 4:.

In Fig. '7 the lower end of the characteristic of tube I is compensatedby the tube 5 in the same manner as explained in connection with Fig. 3;the grid of tube 5 being connected to a tap along the resistancecoupling I6 and the cathode of tube 5 being connected to the same end ofresistor l6 as is the grid of tube I through resistor I. As in Fig. 4,resistor 20 is included in the plate circuit of tube I to apply a degreeof compensation to the upper end of the characteristic; the cathode oftube I being connected to the other end of resistor I6 through thisresistor 2d. The potential difierence across resistor 25 due to flow ofcurrent in the output adv circuit is in a direction to reduce thepositive grid excitation voltage and also, of course, to cut down theeflective plate potential with in creasing plate current thusintroducing some compensation into both grid and plate circuits.

In Fig. 8 is illustrated a circuit in which a single compensating tubein cooperation with a four-element full wave rectifier (which may be ofthe metal oxide type) compensates both ends of the mutual characteristicof two tubes of a push-pull amplifier. The two tubes I and la whosecharacteristics are to be compensated have their grids connected toopposite ends of the secondary Ila of input transformer I'i', theircathodes connected together and to the midpoint of secondary IIa throughthe compensating resistor I and their plates connected to opposite endsof the primary of the output transalternating current terminals of afull wave rectifier indicated diagrammatically at 2I. A resistor 22 isconnected across the direct current terminals of rectifier 2|. The gridof the single compensating tube 5 is connected, through grid biasingbattery 9, with one end of resistor 22,

and the cathode of this tube is connected to the other end of thisresistor. The cathode of tube 5 is connected to the cathodes of tubes Iand la and one end of resistor l, and the plate of tube 5 is connected,through the plate battery I5 to the other, or grid end of this resistor.

The specific connections of resistor 22 to the rectifier 2I and theparticular value selected for battery 9 depend upon the type ofamplifier circuit with which the compensating circuit is to be employed.When used with a class A amplifier, as compensation is desired wheneverthe instantaneous excitation voltage departs from zero in eitherdirection, the grid end of resistor 22 is connected to the positivedirect current terminal of the rectifier as shown in Fig. 8a, andbattery 9 is so chosen as to insure substantially no current throughresistor I when the potential difierence across resistor 22 is zero.Thus, de-

parture from zero excitation voltage in either that maximum currentflows through resistor I; r

when the instantaneous excitation voltage is zero, as in this class ofcircuit the nearer to cut off that tubes I and la can be biased, withoutdistortion the better, and the connection and adjustment above indicatedpermit or" a greater negative biasing of tubes I and Ia by theintroduction of an additional negative bias during approximately zeroinstantaneous voltage which is gradually removed as the excitationvoltage. increases positively for either tube.

The compensating circuit of Fig. 8 may be used, of course, for doubleend compensation of one instead of two tubes if desired and when so usedmay be readily modified, as shown in Fig. 9, to give a greater degree ofcompensation on one end of the mutual characteristic than on the other.To effect this difference in compensation a resistor 23 may be insertedin one arm of the rectifier 2! to vary the potential difference acrossresistor 22, and. consequently the plate current of tube 5, withopposite halves of the excitation voltage wave. With the connectionsillustrated in Fig. 9 when the excitation voltage across tube I ispositive no current flows through resistor 23, but when the excitationvoltage applied to tube is negative current flows through resistor 23and consequently the voltage across resistor 22 is relatively lessduring the negative half cycle of the impressed Voltage. Except when aresistance 23 is used in one arm of the rectifier circuit the resistance22, although it may be used, is theoretically unnecessary.

It will be understood that the circuit of Fig. 9

is but one possible arrangement for obtaining unequal compensation inthe two ends of a mutual characteristic. In Fig. 5, for example, thissame result could be obtained by insertion of a resistance in one or theother of the plate circuits of tubes 5 and 5a, and in Fig. '7 the samegeneral result may be obtained by proper choice of values for resistorsl and 29.

As will be readily appreciated, a two-element instead of a four-elementrectifier could be used in the circuit of either Fig. 8 or Fig. 9; thetwo elements being connected across the winding [8 and having theiroutput sides connected through resistor 22 with the mid-point of winding18, if equal compensation at both ends of the mutual characteristic isdesired, or with winding l8 at a point suitably removed from themidpoint or with a resistance in one of the A. C. leads if unequalcompensation of the two ends of the characteristic is desired.

The combination of a full wave rectifier and a single compensating tubeis thus capable of performing all of the functions of two compensatingtubes in straightening both ends of the mutual characteristics of one ortwo tubes of a class A amplifier and, in addition, is capable ofimproving the operation of class B amplifiers by providing a moredefinite cut-off point on the lower end of the mutual characteristic.

Various circuits have now been described, with particular reference totheir utility in so distorting the grid excitation voltage of an audiofrequency amplifier as to increase the range of operation and theefficiency and output of the tube by, in effect, increasing the linearportion of the mutual characteristic thereof. The method and circuitsdisclosed may be used for compensating the mutual characteristic of anymultiple element tube, whether of the type illustrated or of any othertype as, for example, one having a screen grid. As heretofore indicatedin connection with Figs. 8 and 9, the new method and the new circuitsdescribed may be used in connection with other than audio amplifiers.Furthermore, a circuit such as that shown in Fig. 1 for example could beused to deliver a frequency just twice that of the input voltage. Forthis purpose (considering one half cycle) the grid bias for thecompensating tube is so chosen as to permit appreciable current to flowin the plate circuit thereof during only half of each (half) cycle andthe value of the resistance 7 is so chosen that the potential differencethereacross when maximum current fiows in the plate circuit of thecompensating tube is exactly equal to the positive grid excitationvoltage for tube I. Under these conditions, with proper phase relation,the potential drop across the resistance '1 neutralizes the excitationvoltage for tube l at its peak and thus causes tube l to respond as ifexcited by a frequency twice that impressed upon the input transformer.Also, as mentioned heretofore, any one of the circuits could be used ifdesired to increase, rather than decrease, the distortion by merelyreversing connections to the compensating resistance. The new circuitsor others adapted for the practice of the method may be advantageouslyemployed in any one, several or all stages of a multi-stage fundamentalcircuit. As compared with methods heretofore employed for decreasingdistortion in electron tubes the present method is superior in that itdoes not depend on uncertain and widely varying grid characteristics ofthe tubes and in that it can be used in a circuit in which the gridsdraw no current. In all the circuits except Figs. 6, 8 and 9 thecompensation is shown introduced directly in the grid lead, theconventional way. If, however, the compensation be introduced in thecathode lead (and it makes no difference, since it is a part of the gridcircuit) a common source of filament supply can be used for both thefundamental and compensating tubes. This is the case in Fig. 6; howeverto use Fig. 6 with a common filament battery it would be necessary tomove the plate battery [5 from the location indicated, inserting itbetween the two connections on the extreme right of resistance 1. Allfilaments in the circuit would then be at the same potential.

The following is claimed:

1. The method of distorting the wave form of the voltage applied to thegrid of a vacuum tube which comprises rectifying a voltage varying withthat to be distorted, exciting a compensating tube with theinstantaneous rectified voltage and utilizing variations in the currentdelivered by the compensating tube to vary the voltage applied to thegrid of the first mentioned tube, the rectified voltage being impressedas a positive voltage to the compensating tube and the compensating tubebeing so biased as to yield substantially no current when theinstantaneous excitation voltage is zero.

2. The method of distorting the wave form of the voltage applied to thegrid of a vacuum tube which comprises rectifying a voltage varying withthat to be distorted, exciting a compensating tube with theinstantaneous rectified voltage and utilizing variations in the currentdelivered by the compensating tube to vary the voltage applied to thegrid of the first mentioned tube, the rectified voltage being impressedas a negative voltage to the compensating tube and the compensating tubebeing so biased as to yield maximum current when the excitation voltageis zero.

3. The method of distorting the wave form of the voltage applied to thegrid of a vacuum tube which comprises rectifying a voltage varying withthat to be distorted, exciting a complied to the first-mentioned tubeduring opposite half cycles of the excitation voltage.

4. An electric circuit including in combination an amplifier tube, afull wave rectifier, a compensating tube connected in the output circuitof the rectifier, a source of excitation voltage for said amplifier andsaid rectifier, a resistor in the output circuit of said compensatingtube and so connected in the input circuit of the amplifier tube as tosuperpose an increasing negative voltage upon the excitation voltage forthe amplifier tube as the output current of said compensating tubeincreases.

5. An electric circuit according to claim 4 wherein the grid of saidcompensating tube is connected to the positive direct current terminalof said rectifier and such grid biasing means are provided for saidcompensating tube as to cause substantially no current to flow throughsaid resistor when the instantaneous excitation voltage is zero wherebydouble end compensation of the mutual characteristic of said amplifiertube is obtained.

6. An electric circuit according to claim 4 wherein the grid of saidcompensating tube is connected to the negative direct current terminalof said rectifier and such grid biasing means are provided for saidcompensating tube as to cause maximum current to flow through saidresistor when the instantaneous excitation voltage is zero.

7. An electric circuit comprising in combina tion a source of excitationvoltage a pair of amplifiers connected in push-pull arrangement forexcitation by said source, a resistor common to the input circuits ofsaid amplifiers, a rectifier connected to said source, a compensatingtube having its control grid connected to one direct current terminal ofsaid rectifier and its cathode connected to the other direct currentterminal of said rectifier, said resistor being so connected in theoutput circuit of said compensating tube as to superpose an increasingnega tive voltage upon the excitation voltage for said amplifiers as theoutput current of said compensating tube increases.

8. An electric circuit according to claim 7 wherein the control grid ofsaid compensating tube is connected to the positive direct currentterminal of said rectifier whereby minimum current flows in the platecurrent of said compensating tube when the instantaneous value of theexcitation voltage is zero and both ends of the mutual characteristicsof both amplifier tubes are compensated.

9. An electric circuit according to claim 7 wherein the control grid ofsaid compensating tube is connected to the negative direct currentterminal of said rectifier whereby maximum current flows in the outputcircuit of said compensating tube when the instantaneous value of theexcitation voltage is zero permitting adjustment of the bias of saidpair of amplifiers closer to the cut-off values.

10. An electric circuit for compensating both ends of the mutualcharacteristic of an amplifier tube comprising in combination a sourceof alternating excitation voltage for said amplifier tube, a resistor inthe input circuit of the tube, means for causing a unidirectionalcurrent varying with the magnitude of the excitation voltage to flowthrough said resistor when the instantaneous excitation voltage of saidsource departs from zero in either direction, said last mentioned meansincluding at least one compensating tube having said resistor in theoutput circuit thereof and including a second resistor adapted to varythe relation between the current through said first mentioned resistorand the magnitude of the instantaneous excitation voltage with oppositehalf cycles of the excitation voltage, the direction of current throughsaid first mentioned resistor being such as to cause the potentialdifierence thereacross to operate as a superposed negative voltage tothe excitation voltage in said tube.

11. A circuit according to claim 10 wherein said means for causingcurrent to flow through said first mentioned resistor include a fullwave metal oxide rectifier connected between said source and saidcompensating tube to impress upon said compensating tube a varyingdirect voltage when the excitation voltage departs from zero saidrectifier being arranged to impress upon said compensating tube arelatively greater voltage during one half cycle of the excitationvoltage than during the other half cycle thereof.

12. An electric circuit according to claim 10 wherein said means forcausing current to flow through said first mentioned resistor include afour-element full wave rectifier across the alternating currentterminals of which a voltage varying with the excitation voltage isimpressed and across the direct current terminals of which the inputterminals of said compensating tube are connected, said second resistorbeing included in series with one element of said rectifier whereby thevoltage impressed across said compensating tube depends upon the phaseas well as the magnitude of the excitation voltage.

RICHARD P. BALLOU.

